On our tug, the front tire spec'd is a 110/90-18, with a nominal OD of 25.8"

The Heidenau K-28, a square profile sidecar tire, is 4.00-18, with a nominal OD 26.4.

Even though the K-28 is specified for a slightly narrower rim width, and has a slightly larger OD, I'm thinking that this won't be a big deal, and that the OD once fitted to the rim will be roughly the same, the K-28 should work just fine, and will be better for the application than a 'straight' M/C tire.

Had one of those Custom Chrome front fenders on my '87 FXLR/Springer conversion. Sure kept the crotch dry in the rain.
Later I dumped the primitive poorly made Sputnik for a more comfortable Velorex 562.

K-28 (tire) showed up right after Thanksgiving, mounted it on New Year's Day. Close fit, but didn't need to move the front fender.

Took it out today and WOW! No more low speed head shake (even without ballast), no notable increase in turning effort, and picked up 1/2" extra ground clearance in front. Even new, cornering grip is improved as well.